1. Field of the Invention
The present invention relates to scheduling methods in wireless communication networks.
2. Description of the Related Art
Presently, base stations implement scheduling algorithms to schedule transmission by mobile subscribers separately and independently of the beam forming approach, which may also be implemented by the base station. First, scheduling of mobile stations for transmission will be discussed and then different beam forming concepts will be discussed.
A base station typically includes a scheduler implementing a scheduling algorithm that establishes an order in which to schedule the mobile stations for transmission. For example, it is known for the scheduler to establish a priority vector S, an example of which is shown below in Equation 1.S=[Sk, Sl . . . Sz]]  (1)where Si is a service metric value for mobile station i.
The priority vector S is a sorted list of service metric values for the mobile stations to be scheduled, with each value being associated with one of the mobile stations. For example, the service metric value may represent the data history of the mobile user, a channel state, user priority based on a subscribed quality of service, etc. The scheduler sorts the values; for example, from lowest to highest, to establish a scheduling priority for the mobile stations. The mobile associated with the lowest service metric value corresponds to the highest priority mobile station and will be scheduled first. As shown in Equation 1, Sk corresponds to the lowest service metric value, and therefore, mobile station k will be scheduled first.
The scheduler of a base station may apply various scheduling algorithms in determining the service metric value for each mobile station; for example, an algorithm to generate a service metric value may combine one or more measurement values to derive a service metric value. Also, scheduling algorithms may sort or order the scheduling priority of the mobile stations in various ways. For example, these scheduling algorithms may include a round robin scheduling algorithm, a proportional fair scheduling algorithms, as well as other well-known scheduling algorithms.
The quality of service the base station provides to a mobile station may be affected by the scheduling algorithm as will be appreciated from the above description, but may also be affected by the beam forming approach adopted by the base station. Typically, a base station employs one of two types of beam forming: fixed beam forming and adaptive beam forming. In fixed beam forming, the base station uses a number of preformed antenna beams to cover the area of cell. Each mobile station is assigned to one of the preformed beams. FIG. 1 illustrates a base station 10 having four antenna elements in a uniform linear array to form four transmit beams in a sector of the cell serviced by the base station 10. A base station using fixed beam forming may suffer from performance degradation due to interference when a plurality of scheduled mobile stations are assigned to a same one of the beams.
In adaptive beam forming, the base station provides a individual transmit beam for each mobile station; namely, as many beams as mobile stations are formed. This approach provides gains, such as in signal-to-noise ratio and therefore throughput, when the mobile stations are separated by a certain angular distance. However, scheduled mobile stations may suffer from performance degradation due to interference when this angular distance is less than a threshold angle.
Unfortunately, existing scheduling algorithms do not account for the detrimental impact interference may have as a result of the beam forming approach implemented at the base station.